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Abstract:

Methods for forming a TCO layer on a substrate are generally provided and
include sputtering a TCO layer on a substrate from a target including
cadmium stannate. A cap material (e.g., including cadmium) is deposited
onto an outer surface of an indirect anneal system, and the TCO layer can
be annealed at an anneal temperature while in contact with or within
about 10 cm of the cap material.
An anneal oven is also generally provided and includes an indirect anneal
system defining a deposition surface and an anneal surface such that a
cap material deposited on the anneal surface of the indirect anneal
system is positioned to be in contact with or within about 10 cm of a
thin film on the substrate. A cap material source can be positioned to
deposit the cap material onto the deposition surface such that the anneal
surface comprises the cap material.

Claims:

1. A method for forming a transparent conductive oxide layer on a
substrate, the method comprising: sputtering a transparent conductive
oxide layer on a substrate from a target, wherein the target comprises
cadmium stannate, and wherein the transparent conductive oxide layer is
sputtered at a sputtering temperature of about 10.degree. C. to about
100.degree. C.; depositing a cap material onto an outer surface of an
indirect anneal system, wherein the cap material comprises cadmium; and,
annealing the transparent conductive oxide layer at an anneal temperature
of about 500.degree. C. to about 700.degree. C. while in contact with or
within about 10 cm of the cap material deposited on the outer surface of
the indirect anneal system.

3. The method as in claim 1, wherein the cap material consists of cadmium
sulfide.

4. The method as in claim 1, wherein a continuous belt defines the outer
surface of the indirect anneal system such that the cap material is
deposited onto the continuous belt.

5. The method as in claim 4, wherein the conveyor belt has a tension
configured to allow the conveyor belt to contact the transparent
conductive oxide layer during annealing.

6. The method as in claim 4, wherein the indirect anneal system further
comprises a tension control system configured to adjust the distance that
the conveyor belt is from the transparent conductive oxide layer.

7. The method as in claim 6, wherein the tension control system comprises
a plurality of rollers, wherein at least one roller is moveable to adjust
the tension in the conveyor belt.

8. The method as in claim 1, wherein the indirect anneal system comprises
a plurality of slats forming a continuous loop such that the plurality of
slats forms a deposition surface and an anneal surface, wherein the cap
material is deposited onto the deposition surface.

9. The method as in claim 8, wherein the continuous loop of the plurality
of slats has a tension configured to allow the anneal surface to contact
the transparent conductive oxide layer during annealing.

10. The method as in claim 1, wherein the transparent conductive oxide
layer is annealed in an annealing atmosphere comprising an inert gas.

11. The method as in claim 10, wherein the annealing atmosphere further
comprises a reducing gas.

13. The method as in claim 1, wherein the anneal temperature is about
550.degree. C. to about 650.degree. C.

14. An anneal oven for annealing a thin film layer on a substrate, the
anneal oven comprising: a transport system configured to carry a
substrate through the anneal oven; a heating element configured to heat
the annealing oven to the annealing temperature; an indirect anneal
system defining a deposition surface and an anneal surface, wherein a cap
material deposited on the anneal surface of the indirect anneal system is
positioned to be in contact with or within about 10 cm of a thin film on
the substrate; and, a cap material source positioned to deposit the cap
material onto the deposition surface such that the anneal surface
comprises the cap material, wherein the cap material source comprises
cadmium.

15. The anneal oven as in claim 14, wherein the indirect anneal system
comprises a continuous belt about at least two rollers, wherein the
continuous belt defines the deposition surface and the anneal surface.

16. The anneal oven as in claim 15, wherein the conveyor belt has a
tension control system configured to adjust the tension in the continuous
belt.

17. The anneal oven as in claim 16, wherein the tension control system
comprises a plurality of rollers, wherein at least one roller is moveable
to adjust the tension in the conveyor belt.

18. The anneal oven as in claim 16, wherein the tension control system
comprises a tension roll positioned between a pair of directional
rollers, wherein the tension roll is moveable to adjust the tension of
the continuous belt.

19. The anneal oven as in claim 14, wherein the indirect anneal system
comprises a plurality of slats forming a continuous loop such that the
plurality of slats forms a deposition surface and an anneal surface.

20. The anneal oven as in claim 19, wherein the continuous loop of the
plurality of slats travels about at least two sprockets, wherein at least
one of the sprockets is moveable to adjust the tension of the continuous
loop of the plurality of slats.

Description:

FIELD OF THE INVENTION

[0001] The subject matter disclosed herein relates generally to forming a
conductive transparent oxide film layer. More particularly, the subject
matter disclosed herein relates to apparatus and methods of forming a
conductive transparent oxide film layer for use in cadmium telluride thin
film photovoltaic devices.

BACKGROUND OF THE INVENTION

[0002] Thin film photovoltaic (PV) modules (also referred to as "solar
panels") based on cadmium telluride (CdTe) paired with cadmium sulfide
(CdS) as the photo-reactive components are gaining wide acceptance and
interest in the industry. CdTe is a semiconductor material having
characteristics particularly suited for conversion of solar energy to
electricity. For example, CdTe has an energy bandgap of about 1.45 eV,
which enables it to convert more energy from the solar spectrum as
compared to lower bandgap semiconductor materials historically used in
solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe
converts radiation energy in lower or diffuse light conditions as
compared to the lower bandgap materials and, thus, has a longer effective
conversion time over the course of a day or in cloudy conditions as
compared to other conventional materials. The junction of the n-type
layer and the p-type layer is generally responsible for the generation of
electric potential and electric current when the CdTe PV module is
exposed to light energy, such as sunlight. Specifically, the cadmium
telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n
heterojunction, where the CdTe layer acts as a p-type layer (i.e., a
positive, electron accepting layer) and the CdS layer acts as a n-type
layer (i.e., a negative, electron donating layer).

[0003] A transparent conductive oxide ("TCO") layer is commonly used
between the window glass and the junction forming layers. For example,
the TCO layer may be sputtered from a cadmium stannate (i.e.,
Cd2SnO4) target by either of two processes: hot sputtering or
cold sputtering. When hot sputtered, the TCO layer is typically deposited
at sputtering temperatures above about 250° C. in a one step
sputtering process. When cold sputtered (e.g., at about room
temperature), the TCO layer must be annealed following sputtering of the
layer in a second step to convert the layer from an amorphous layer to a
crystalline layer.

[0004] Though the hot sputtering process is more streamlined (i.e., only
requiring a single step), the hot sputtered TCO layers can have a much
higher resistivity than the cold sputtered TCO layers--even when
sputtered from the same material (e.g., cadmium stannate)--making the hot
sputtered TCO layer less attractive for the end use. Although not wishing
to be bound by any particular theory, it is believed that this difference
in resistivities between the hot sputtered layer and the cold sputtered
layer likely stems from a difference in the as-deposited stoichiometry.
For example, when sputtering from a cadmium stannate target, it is
presently believed that cold sputtering produces a layer having the
stoichiometry Cd2SnO4, which is the desired stoichiometry for
cadmium stannate. However, other processing issues exist that hinders the
viability of cold sputtering to form the TCO layer, especially from a
cadmium stannate target. For example, the annealing process can sublimate
cadmium atoms off of the TCO layer, altering the stoichiometry of the TCO
layer, especially along its outer surface.

[0005] To counteract this loss of cadmium atoms from the surface of the
TCO layer, the TCO layer is typically annealed in contact with an anneal
plate containing cadmium. For instance, an anneal plate having cadmium
sulfide on its surface contacting the TCO layer can be used to provide
additional cadmium to the TCO layer during annealing to inhibit the loss
of cadmium from the TCO layer.

[0006] However, contact plate is awkward for manufacturing use in a large
commercial-scale manufacturing setting, and becomes depleted of cadmium
during repeated use, requiring plate change. As such, the use of such
anneal plates adds manufacturing processes and materials, effectively
increasing the manufacturing cost and complexity for formation of the PV
modules. Thus, a need exists for methods of forming a TCO layer having
the conductivity of cold sputtered layers with the processing ease found
in those hot sputtered layers.

BRIEF DESCRIPTION OF THE INVENTION

[0007] Aspects and advantages of the invention will be set forth in part
in the following description, or may be obvious from the description, or
may be learned through practice of the invention.

[0008] Methods are generally provided for forming a conductive oxide layer
on a substrate. In one particular embodiment, the method can include
sputtering a transparent conductive oxide layer on a substrate from a
target including cadmium stannate at a sputtering temperature of about
10° C. to about 100° C. A cap material (e.g., including
cadmium) can be deposited onto an outer surface of an indirect anneal
system, and the transparent conductive oxide layer can be annealed at an
anneal temperature of about 500° C. to about 700° C. while
in contact with or within about 10 cm of the cap material deposited on
the outer surface of the indirect anneal system.

[0009] An anneal oven for annealing a thin film layer on a substrate is
also generally provided. The anneal oven can include a transport system
configured to carry a substrate through the anneal oven and a heating
element configured to heat the annealing oven to the annealing
temperature. An indirect anneal system defining a deposition surface and
an anneal surface can also be included within the anneal oven such that a
cap material deposited on the anneal surface of the indirect anneal
system is positioned to be in contact with or within about 10 cm of a
thin film on the substrate. A cap material source can be positioned to
deposit the cap material onto the deposition surface such that the anneal
surface comprises the cap material.

[0010] These and other features, aspects and advantages of the present
invention will become better understood with reference to the following
description and appended claims. The accompanying drawings, which are
incorporated in and constitute a part of this specification, illustrate
embodiments of the invention and, together with the description, serve to
explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0011] A full and enabling disclosure of the present invention, including
the best mode thereof, directed to one of ordinary skill in the art, is
set forth in the specification, which makes reference to the appended
figures, in which:

[0012] FIG. 1 shows a general schematic of a cross-sectional view of an
exemplary cadmium telluride thin film photovoltaic device according to
one embodiment of the present invention;

[0013] FIG. 2 shows a flow diagram of an exemplary method of manufacturing
a photovoltaic module including a cadmium telluride thin film
photovoltaic device;

[0014] FIG. 3 shows a general schematic of a cross-sectional view of an
exemplary sputtering chamber according to one embodiment of the present
invention; and,

[0015] FIG. 4 shows a general schematic of an exemplary anneal oven with a
cap material defining a surface of a conveyor; and,

[0016] FIG. 5 shows a general schematic of another exemplary anneal oven
with a cap material defining a surface of a conveyor belt.

[0017] Repeat use of reference characters in the present specification and
drawings is intended to represent the same or analogous features or
elements.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Reference now will be made in detail to embodiments of the
invention, one or more examples of which are illustrated in the drawings.
Each example is provided by way of explanation of the invention, not
limitation of the invention. In fact, it will be apparent to those
skilled in the art that various modifications and variations can be made
in the present invention without departing from the scope or spirit of
the invention. For instance, features illustrated or described as part of
one embodiment can be used with another embodiment to yield a still
further embodiment. Thus, it is intended that the present invention
covers such modifications and variations as come within the scope of the
appended claims and their equivalents.

[0019] In the present disclosure, when a layer is being described as "on"
or "over" another layer or substrate, it is to be understood that the
layers can either be directly contacting each other or have another layer
or feature between the layers. Thus, these terms are simply describing
the relative position of the layers to each other and do not necessarily
mean "on top of" since the relative position above or below depends upon
the orientation of the device to the viewer. Additionally, although the
invention is not limited to any particular film thickness, the term
"thin" describing any film layers of the photovoltaic device generally
refers to the film layer having a thickness less than about 10
micrometers ("microns" or "μm").

[0020] It is to be understood that the ranges and limits mentioned herein
include all ranges located within the prescribed limits (i.e.,
subranges). For instance, a range from about 100 to about 200 also
includes ranges from 110 to 150, 170 to 190, 153 to 162, and 145.3 to
149.6. Further, a limit of up to about 7 also includes a limit of up to
about 5, up to 3, and up to about 4.5, as well as ranges within the
limit, such as from about 1 to about 5, and from about 3.2 to about 6.5.

[0021] Methods are generally disclosed for cold sputtering a transparent
conductive oxide layer ("TCO layer") on a substrate followed by annealing
the TCO layer in contact with or in close proximity to a cadmium
containing cap material of a conveyor belt or roll. The cold sputtered
TCO layer can include cadmium, such as a TCO layer including cadmium
stannate (i.e., Cd2SnO4) or stoichiometric variation of
cadmium, tin, and oxygen. Other materials may also be present in the TCO
layer, including other oxides (e.g., tin oxide, zinc oxide, or indium tin
oxide, or mixtures thereof). Additionally, the TCO layer can include
other conductive, transparent materials.

[0022] In one particular embodiment, the TCO layer can be formed by
sputtering (e.g., DC sputtering or RF sputtering) on the substrate from a
target (e.g., a target including cadmium stannate or a target including
an alloy of cadmium and tin). For example, a cadmium stannate layer can
be formed by sputtering a hot-pressed target containing cadmium stannate
(Cd2SnO4) and/or stoichiometric amounts of SnO2 and CdO
onto the substrate in a ratio of about 1 to about 2. Alternatively, a
cadmium stannate layer can be formed by sputtering a metal target of
cadmium and tin in an oxidizing atmosphere (e.g., a sputtering atmosphere
comprising oxygen at about 100% by volume).

[0023] The sputtering temperature for forming the TCO layer containing
cadmium stannate is relatively low, so that the sputtering method can be
referred to as "cold sputtering." For example, the sputtering temperature
can be about 10° C. to about 100° C., such as about
20° C. to about 50° C. In one particular embodiment, the
sputtering temperature can be room temperature (e.g., about 20° C.
to about 25° C.).

[0024] After cold sputtering of the TCO layer onto the substrate, the TCO
layer is annealed to re-crystallize the TCO layer into a more uniform
thin film layer on the substrate in contact with or in close proximity to
a cadmium containing cap material of a conveyor belt or roll. Without
wishing to be bound by any particular theory, it is believed that the
cadmium containing cap material on a conveyor belt or roll can provide
cadmium atoms to the annealing atmosphere and/or to the surface of the
TCO layer via sublimation from the cadmium containing cap material and
can inhibit sublimation of cadmium atoms from the surface of the TCO
layer during the annealing process. For instance, the cadmium containing
cap material may form an equilibrium-type relationship with the surface
of the substrate (i.e., the TCO layer) forcing more cadmium atoms to
remain in the TCO layer during the annealing process. Additionally, the
cadmium containing cap material can provide an increased stoichiometric
amount of cadmium to the surface of the TCO layer, helping to maintain or
even increase the cadmium content of the cold sputtered TCO layer.

[0025] Cadmium can constitute any portion of the total annealing pressure
suitable to inhibit cadmium from sublimating off of the surface of the
substrate during annealing of the TCO layer. For example, cadmium can
constitute about 1% to about 50% by volume of the annealing atmosphere,
such as about 5% to about 25% by volume.

[0026] The annealing atmosphere can include inert gases (e.g., argon,
etc.); however, other gases may be present. In one particular embodiment,
a reducing gas may be included in the annealing atmosphere. A reducing
gas may help improve conductivity of the TCO layer by increasing the
oxygen vacancies in a TCO layer of cadmium stannate. On particularly
suitable reducing gas is hydrogen sulfide (H2S), although any
reducing gas may be used such as hydrogen gas (H2), etc. For
example, a reducing gas may constitute about 1% to about 25% by volume of
the annealing atmosphere, such as about 5% to about 15% by volume.

[0027] In one embodiment, the annealing atmosphere may be substantially
free from other reactive gasses, such as oxygen, nitrogen, and/or
halogen-containing gasses. As used herein, the term "substantially free"
means no more than an insignificant trace amount present and encompasses
completely free (e.g., 0% up to about 0.0001% by volume of the annealing
atmosphere).

[0028] The overall pressure of the annealing atmosphere during annealing
can be from about 50 mTorr to about 1000 Torr (e.g., from about 1 Torr to
about 850 Torr, such as from about 10 Torr to about 800 Torr). In one
particular embodiment, the overall pressure of the annealing atmosphere
can be about room pressure (e.g., about 760 Torr).

[0029] After annealing in the presence of cadmium, any excess cadmium
deposited onto the substrate during annealing can be subsequently etched
from the TCO layer. For example, a chemical etch can be used to remove
excess cadmium deposited during annealing from the surface of the TCO
layer. In one particular embodiment, the TCO layer can be cooled after
annealing and then chemically etched to remove any excess deposited
cadmium from its surface. The etchant can be chosen to provide selective
removal of cadmium without substantially affecting the TCO layer.
Suitable selective etchants are those that can remove cadmium without
substantially removing or otherwise affecting the underlying TCO layer
(e.g., cadmium stannate). Particular selective etchants can include, but
are not limited to, hydrochloric acid, nitric acid, or mixtures thereof.

[0030] As stated the TCO layer can be formed via sputtering. Sputtering
deposition generally involves ejecting material from a target, which is
the material source, and depositing the ejected material onto the
substrate to form the film. DC sputtering generally involves applying a
direct current to a metal target (i.e., the cathode) positioned near the
substrate (i.e., the anode) within a sputtering chamber to form a
direct-current discharge. The sputtering chamber can have a reactive
atmosphere (e.g., an oxygen atmosphere, nitrogen atmosphere, fluorine
atmosphere) that forms a plasma field between the metal target and the
substrate. Other inert gases (e.g., argon, etc.) may also be present. The
pressure of the reactive atmosphere can be between about 1 mTorr and
about 20 mTorr for magnetron sputtering. The pressure can be even higher
for diode sputtering (e.g., from about 25 mTorr to about 100 mTorr). When
metal atoms are released from the target upon application of the voltage,
the metal atoms deposit onto the surface of the substrate. For example,
when the atmosphere contains oxygen, the metal atoms released from the
metal target can form a metallic oxide layer on the substrate. The
current applied to the source material can vary depending on the size of
the source material, size of the sputtering chamber, amount of surface
area of substrate, and other variables. In some embodiments, the current
applied can be from about 2 amps to about 20 amps.

[0031] Conversely, RF sputtering involves exciting a capacitive discharge
by applying an alternating-current (AC) or radio-frequency (RF) signal
between the target (e.g., a ceramic source material) and the substrate.
The sputtering chamber can have an inert atmosphere (e.g., an argon
atmosphere) which may or may not contain reactive species (e.g., oxygen,
nitrogen, etc.) having a pressure between about 1 mTorr and about 20
mTorr for magnetron sputtering. Again, the pressure can be even higher
for diode sputtering (e.g., from about 25 mTorr to about 100 mTorr).

[0032] FIG. 3 shows a general schematic as a cross-sectional view of an
exemplary DC sputtering chamber 60 according to one embodiment of the
present invention. A DC power source 62 is configured to control and
supply DC power to the chamber 60. As shown, the DC power source applies
a voltage to the cathode 64 to create a voltage potential between the
cathode 64 and an anode formed by the chamber wall, such that the
substrate is in between the cathode and anode. The glass substrate 12 is
held between top support 66 and bottom support 67 via wires 68 and 69,
respectively. Generally, the glass substrate is positioned within the
sputtering chamber 60 such that the TCO layer 14 is formed on the surface
facing the cathode 64.

[0033] A plasma field 70 is created once the sputtering atmosphere is
ignited, and is sustained in response to the voltage potential between
the cathode 64 and the chamber wall acting as an anode. The voltage
potential causes the plasma ions within the plasma field 70 to accelerate
toward the cathode 64, causing atoms from the cathode 64 to be ejected
toward the surface on the glass substrate 12. As such, the cathode 64 can
be referred to as a "target" and acts as the source material for the
formation of the TCO layer 14 on the surface facing the cathode 64. The
cathode 64 can be a metal alloy target, such as elemental tin, elemental
zinc, or mixtures thereof. Additionally, in some embodiments, a plurality
of cathodes 64 can be utilized. A plurality of cathodes 64 can be
particularly useful to form a layer including several types of materials
(e.g., co-sputtering). Since the sputtering atmosphere contains oxygen
gas, oxygen particles of the plasma field 70 can react with the ejected
target atoms to form an oxide layer on the TCO layer 14 on the glass
substrate 12.

[0034] Although only a single DC power source 62 is shown, the voltage
potential can be realized through the use of multiple power sources
coupled together. Additionally, the exemplary sputtering chamber 60 is
shown having a vertical orientation, although any other configuration can
be utilized.

[0035] For example, the TCO layer can be formed via sputtering at the
specified sputtering temperature (i.e., cold sputtering) from a cadmium
stannate target to form a cadmium stannate TCO layer on the substrate.
After sputtering the TCO layer, the substrate with the TCO layer can then
be transferred into an anneal oven.

[0036] Each of FIGS. 4 and 5 shows a side-view diagram of an exemplary
annealing oven 100. A substrate 12 can pass from the sputtering chamber
and into the anneal chamber 100 though the slit 101, past the indirect
anneal system 124, and out of the anneal chamber 100 through the slit
103. The substrate 12 is shown traveling along rollers 105 through the
anneal oven 100, although any carrier system can be utilized to transport
the substrate 12 though the anneal oven 100 (e.g., a conveyor system,
etc.). The sputtering chamber can be adjacent to the anneal chamber 100
or may be separated from the anneal chamber 100, as long as the
sputtering atmosphere in the sputtering chamber is separated from the
annealing atmosphere in the anneal chamber 100.

[0037] The anneal oven 100 can be connected to a vacuum pump 102 to
control the annealing pressure of the annealing atmosphere within the
anneal oven 100. Heating elements 104 are configured to heat the
annealing atmosphere within the anneal oven 100 to the desired annealing
temperature (e.g., about 500° C. to about 700° C.), which
can in-turn heat the substrate 12 and the TCO layer 14 to the anneal
temperature. Although shown inside the anneal oven 100, any heating
element 104, positioned inside or outside of the anneal oven can be used
to provide heat to the anneal oven 100. In one particular embodiment, the
inner walls 106 of the anneal oven 100 can be hot oven walls (i.e.,
heated to about the annealing temperature) to provide heat to the
annealing atmosphere inside the anneal oven 100. Additionally, any number
of heating elements 104 can be utilized in conjunction with the anneal
oven 100.

[0038] Source gas lines 107, 108 can supply various gases to the anneal
oven 100, while valves 110, 111 can control the gas flow rate of a
particular gas through the source gas lines 107, 108 from a gas source
113, 114, respectively, into the anneal oven 100. For example, a first
source gas line 107 controlled by valve 110 can regulate the amount of
inert gas in annealing atmosphere of the anneal oven 100 supplied from
the cadmium source tank 113, while a second source gas line 108
controlled by valve 111 can regulate the amount of optional gas (e.g., a
reducing gas, an oxidizing gas, etc.) in the annealing atmosphere of the
anneal oven 100 supplied from the inert gas source tank 114. Of course,
any suitable design of an anneal oven can be utilized in accordance with
the present disclosure, including any number of suitable source gas lines
and gas sources.

[0039] FIG. 4 shows a indirect anneal system 124 driven in an endless loop
within the anneal oven 100 around sprockets 126, with the endless loop
having a bottom leg that moves in a conveyance direction of the
substrates 12 through the anneal oven 100, and an upper leg that moves in
an opposite return direction past a cadmium deposition head. The indirect
anneal system 124 includes a plurality of interconnected slats 128, each
defining flat outer surface. Thus, the interconnected slats 128 form a
planar deposition surface 129 defined by adjacent slats 128 along the
upper leg of the indirect anneal system 124 such that the outer surfaces
of the slats 128 lie in a common horizontal plane and define an
uninterrupted deposition surface for the deposition of the cap material
including cadmium. A cap material is shown in FIG. 4 being deposited via
sputtering onto the planar deposition surface 129 defined by adjacent
slats 128 along the upper leg of the indirect anneal system 124 from a
target 120 and the plasma 122.

[0040] The interconnected slats 128 also form a planar anneal surface 130
defined by adjacent slats 128 along the bottom leg of the indirect anneal
system 124 such that the outer surfaces of the slats 128 lie in a common
horizontal plane and define an uninterrupted annealing surface. As the
substrates 12 pass into the anneal oven 100, the TCO layer 14 on the
substrate 12 can be annealed within close proximity to or in contact with
a cap material including cadmium on the planar anneal surface 130.

[0041] A tension control 127 can move one or both of the sprockets 126
horizontally to adjust the tension in the indirect anneal system 124
(i.e., out to increase tension or in to decrease tension), in turn
controlling the distance that the anneal surface 130 is from the outer
surface of the TCO layer 14 on the substrate 12.

[0042] For instance, in one embodiment, the planar anneal surface 130 can
contact the TCO layer 14 during annealing or can be in close proximity to
the TCO layer 14 during annealing, such as about 10 centimeters (cm) or
closer. For example, the planar anneal surface 130 can be about 1
millimeter (mm) to about 5 cm from the surface of the TCO layer, such as
about 5 mm to about 2.5 cm. In one particular embodiment, the planar
anneal surface 130 can rest on top of the TCO layer 14 and move in unison
with the substrate 12 as it travels along rollers 105 through the anneal
oven 100.

[0043] The slats 128 can be configured to withstand the anneal
temperatures and the deposition of the cap material. For example, the
slats 128 can be constructed from graphite, a ceramic (e.g., alumina,
etc.), quartz, high temperature metallic materials (e.g., molybdenum,
titanium, or alloys thereof), and the like.

[0044] FIG. 5 shows another exemplary embodiment of an annealing oven 100
that is similar to that shown in FIG. 4, except for the indirect anneal
system 124. As shown in FIG. 5, the indirect anneal system 124 includes a
continuous belt 132 around rollers 136. The continuous belt 132 defines a
deposition surface 133 on an upper leg of the indirect anneal system 124
configured to receive the cap layer deposited via sputtering from the
target 120 and plasma 122. The continuous belt 132 also defines an anneal
surface 134 on the bottom leg of the indirect anneal system 124. As the
substrates 12 pass into the anneal oven 100, the TCO layer 14 on the
substrate 12 can be annealed within close proximity to or in contact with
the cap material on the surface of the continuous belt 132 including
cadmium on the anneal surface 134.

[0045] A tension control system 138 can adjust the tension in the indirect
anneal system 124, in turn controlling the distance that the anneal
surface 134 is from the outer surface of the TCO layer 14 on the
substrate 12. As shown, a tension roll 140 can move vertically between
two directional rollers 139. As the tension roll 140 moves, the tension
of the continuous belt 132 can be adjusted (i.e., up to decrease tension
and down to increase tension). In one embodiment, the anneal surface 134
can contact the TCO layer 14 during annealing or can be in close
proximity to the TCO layer 14 during annealing, such as about 10 cm or
closer. For example, the anneal surface 134 can be about 1 mm to about 5
cm from the surface of the TCO layer, such as about 5 mm to about 2.5 cm.
In one particular embodiment, the anneal surface 134 can rest on top of
the TCO layer 14 and move in unison with the substrate 12 as it travels
along rollers 105 through the anneal oven 100.

[0046] The continuous belt 132 can be configured to withstand the anneal
temperatures and the deposition of the cap material. For example, the
continuous belt 132 can be constructed from a high-temperature,
non-reactive, flexible material, including but not limited to, austenitic
nickel-chromium-based superalloys (e.g., materials available under the
trade name Inconel® from Special Metals Corporation, Huntington, W.
Va.).

[0047] Although shown utilizing sputtering deposition in each of FIGS. 4
and 5, other deposition methods of depositing a cap material onto the
deposition surface can be utilized. For example, the cap material can be
deposited via close-space sublimation onto the deposition surface, via
spray pyrolysis, chemical vapor deposition, thermal evaporation, etc.

[0048] The presently provided methods of sputtering and annealing a TCO
layer can be utilized in the formation of any film stack that utilizes a
TCO layer, particularly those including a cadmium stannate TCO layer. For
example, the TCO layer can be used during the formation of any cadmium
telluride device that utilizes a cadmium telluride layer, such as in the
cadmium telluride thin film photovoltaic device disclosed in U.S.
Publication No. 2009/0194165 of Murphy, et al. titled "Ultra-high Current
Density Cadmium Telluride Photovoltaic Modules."

[0049] FIG. 1 represents an exemplary cadmium telluride thin film
photovoltaic device 10 that can be formed according to methods described
herein. The exemplary device 10 of FIG. 1 includes a top sheet of glass
12 employed as the substrate. In this embodiment, the glass 12 can be
referred to as a "superstrate," as it is the substrate on which the
subsequent layers are formed even though it faces upward to the radiation
source (e.g., the sun) when the cadmium telluride thin film photovoltaic
device 10 is in use. The top sheet of glass 12 can be a high-transmission
glass (e.g., high transmission borosilicate glass), low-iron float glass,
or other highly transparent glass material. The glass is generally thick
enough to provide support for the subsequent film layers (e.g., from
about 0.5 mm to about 10 mm thick), and is substantially flat to provide
a good surface for forming the subsequent film layers. In one embodiment,
the glass 12 can be a low iron float glass containing less than about
0.015% by weight iron (Fe), and may have a transmissiveness of about 0.9
or greater in the spectrum of interest (e.g., wavelengths from about 300
nm to about 900 nm). In another embodiment, borosilicate glass may be
utilized so as to better withstand high temperature processing.

[0050] The transparent conductive oxide (TCO) layer 14 is shown on the
glass 12 of the exemplary device 10 of FIG. 1. The TCO layer 14 allows
light to pass through with minimal absorption while also allowing
electric current produced by the device 10 to travel sideways to opaque
metal conductors (not shown). For instance, the TCO layer 14 can have a
sheet resistance less than about 30 ohm per square, such as from about 4
ohm per square to about 20 ohm per square (e.g., from about 8 ohm per
square to about 15 ohm per square). In certain embodiments, the TCO layer
14 can have a thickness between about 0.1 μm and about 1 μm, for
example from about 0.1 μm to about 0.5 μm, such as from about 0.25
μm to about 0.35 μm.

[0051] A resistive transparent buffer layer 16 (RTB layer) is shown on the
TCO layer 14 on the exemplary cadmium telluride thin film photovoltaic
device 10. The RTB layer 16 is generally more resistive than the TCO
layer 14 and can help protect the device 10 from chemical interactions
between the TCO layer 14 and the subsequent layers during processing of
the device 10. For example, in certain embodiments, the RTB layer 16 can
have a sheet resistance that is greater than about 1000 ohms per square,
such as from about 10 kOhms per square to about 1000 MOhms per square.
The RTB layer 16 can also have a wide optical bandgap (e.g., greater than
about 2.5 eV, such as from about 2.7 eV to about 3.0 eV).

[0052] Without wishing to be bound by a particular theory, it is believed
that the presence of the RTB layer 16 between the TCO layer 14 and the
cadmium sulfide layer 18 can allow for a relatively thin cadmium sulfide
layer 18 to be included in the device 10 by reducing the possibility of
interface defects (i.e., "pinholes" in the cadmium sulfide layer 18)
creating shunts between the TCO layer 14 and the cadmium telluride layer
22. Thus, it is believed that the RTB layer 16 allows for improved
adhesion and/or interaction between the TCO layer 14 and the cadmium
telluride layer 22, thereby allowing a relatively thin cadmium sulfide
layer 18 to be formed thereon without significant adverse effects that
would otherwise result from such a relatively thin cadmium sulfide layer
18 formed directly on the TCO layer 14.

[0053] The RTB layer 16 can include, for instance, a combination of zinc
oxide (ZnO) and tin oxide (SnO2), which can be referred to as a zinc
tin oxide layer ("ZTO"). In one particular embodiment, the RTB layer 16
can include more tin oxide than zinc oxide. For example, the RTB layer 16
can have a composition with a stoichiometric ratio of ZnO/SnO2
between about 0.25 and about 3, such as in about an one to two (1:2)
stoichiometric ratio of tin oxide to zinc oxide. The RTB layer 16 can be
formed by sputtering, chemical vapor deposition, spraying pryolysis, or
any other suitable deposition method. In one particular embodiment, the
RTB layer 16 can be formed by sputtering (e.g. DC sputtering or RF
sputtering) on the TCO layer 14 (as discussed below in greater detail
with respect to the deposition of the cadmium sulfide layer 18). For
example, the RTB layer 16 can be deposited using a DC sputtering method
by applying a DC current to a metallic source material (e.g., elemental
zinc, elemental tin, or a mixture thereof) and sputtering the metallic
source material onto the TCO layer 14 in the presence of an oxidizing
atmosphere (e.g., O2 gas). When the oxidizing atmosphere includes
oxygen gas (i.e., O2), the atmosphere can be greater than about 95%
pure oxygen, such as greater than about 99%.

[0054] In certain embodiments, the RTB layer 16 can have a thickness
between about 0.075 μm and about 1 μm, for example from about 0.1
μm to about 0.5 μm. In particular embodiments, the RTB layer 16 can
have a thickness between about 0.08 μm and about 0.2 μm, for
example from about 0.1 μm to about 0.15 μm.

[0055] A cadmium sulfide layer 18 is shown on RTB layer 16 of the
exemplary device 10 of FIG. 1. The cadmium sulfide layer 18 is a n-type
layer that generally includes cadmium sulfide (CdS) but may also include
other materials, such as zinc sulfide, cadmium zinc sulfide, etc., and
mixtures thereof as well as dopants and other impurities. In one
particular embodiment, the cadmium sulfide layer may include oxygen up to
about 25% by atomic percentage, for example from about 5% to about 20% by
atomic percentage. The cadmium sulfide layer 18 can have a wide band gap
(e.g., from about 2.25 eV to about 2.5 eV, such as about 2.4 eV) in order
to allow most radiation energy (e.g., solar radiation) to pass. As such,
the cadmium sulfide layer 18 is considered a transparent layer on the
device 10.

[0056] The cadmium sulfide layer 18 can be formed by sputtering, chemical
vapor deposition, chemical bath deposition, and other suitable deposition
methods. In one particular embodiment, the cadmium sulfide layer 18 can
be formed by sputtering (e.g., direct current (DC) sputtering or radio
frequency (RF) sputtering) on the resistive transparent layer 16.
Sputtering deposition generally involves ejecting material from a target,
which is the material source, and depositing the ejected material onto
the substrate to form the film. DC sputtering generally involves applying
a voltage to a metal target (i.e., the cathode) positioned near the
substrate (i.e., the anode) within a sputtering chamber to form a
direct-current discharge. The sputtering chamber can have a reactive
atmosphere (e.g., an oxygen atmosphere, nitrogen atmosphere, fluorine
atmosphere) that forms a plasma field between the metal target and the
substrate. The pressure of the reactive atmosphere can be between about 1
mTorr and about 20 mTorr for magnetron sputtering. When metal atoms are
released from the target upon application of the voltage, the metal atoms
can react with the plasma and deposit onto the surface of the substrate.
For example, when the atmosphere contains oxygen, the metal atoms
released from the metal target can form a metallic oxide layer on the
substrate. Conversely, RF sputtering generally involves exciting a
capacitive discharge by applying an alternating-current (AC) or
radio-frequency (RF) signal between the target (e.g., a ceramic source
material) and the substrate. The sputtering chamber can have an inert
atmosphere (e.g., an argon atmosphere) having a pressure between about 1
mTorr and about 20 mTorr.

[0057] Due to the presence of the resistive transparent layer 16, the
cadmium sulfide layer 18 can have a thickness that is less than about 0.1
μm, such as between about 10 nm and about 100 nm, such as from about
50 nm to about 80 nm, with a minimal presence of pinholes between the
resistive transparent layer 16 and the cadmium sulfide layer 18.
Additionally, a cadmium sulfide layer 18 having a thickness less than
about 0.1 μm reduces any absorption of radiation energy by the cadmium
sulfide layer 18, effectively increasing the amount of radiation energy
reaching the underlying cadmium telluride layer 22.

[0058] A cadmium telluride layer 20 is shown on the cadmium sulfide layer
18 in the exemplary cadmium telluride thin film photovoltaic device 10 of
FIG. 1. The cadmium telluride layer 20 is a p-type layer that generally
includes cadmium telluride (CdTe) but may also include other materials.
As the p-type layer of device 10, the cadmium telluride layer 20 is the
photovoltaic layer that interacts with the cadmium sulfide layer 18
(i.e., the n-type layer) to produce current from the absorption of
radiation energy by absorbing the majority of the radiation energy
passing into the device 10 due to its high absorption coefficient and
creating electron-hole pairs. For example, the cadmium telluride layer 20
can generally be formed from cadmium telluride and can have a bandgap
tailored to absorb radiation energy (e.g., from about 1.4 eV to about 1.5
eV, such as about 1.45 eV) to create the maximum number of electron-hole
pairs with the highest electrical potential (voltage) upon absorption of
the radiation energy. Electrons may travel from the p-type side (i.e.,
the cadmium telluride layer 20) across the junction to the n-type side
(i.e., the cadmium sulfide layer 18) and, conversely, holes may pass from
the n-type side to the p-type side. Thus, the p-n junction formed between
the cadmium sulfide layer 18 and the cadmium telluride layer 20 forms a
diode in which the charge imbalance leads to the creation of an electric
field spanning the p-n junction. Conventional current is allowed to flow
in only one direction and separates the light induced electron-hole
pairs.

[0059] The cadmium telluride layer 20 can be formed by any known process,
such as vapor transport deposition, chemical vapor deposition (CVD),
spray pyrolysis, electro-deposition, sputtering, close-space sublimation
(CSS), etc. In one particular embodiment, the cadmium sulfide layer 18 is
deposited by a sputtering and the cadmium telluride layer 20 is deposited
by close-space sublimation. In particular embodiments, the cadmium
telluride layer 20 can have a thickness between about 0.1 μm and about
10 μm, such as from about 1 μm and about 5 μm. In one particular
embodiment, the cadmium telluride layer 20 can have a thickness between
about 2 μm and about 4 μm, such as about 3 μm.

[0060] A series of post-forming treatments can be applied to the exposed
surface of the cadmium telluride layer 20. These treatments can tailor
the functionality of the cadmium telluride layer 20 and prepare its
surface for subsequent adhesion to the back contact layer(s) 22. For
example, the cadmium telluride layer 20 can be annealed at elevated
temperatures (e.g., from about 350° C. to about 500° C.,
such as from about 375° C. to about 424° C.) for a
sufficient time (e.g., from about 1 to about 10 minutes) to create a
quality p-type layer of cadmium telluride. Without wishing to be bound by
theory, it is believed that annealing the cadmium telluride layer 20 (and
the device 10) converts the normally lightly p-type doped, or even n-type
doped cadmium telluride layer 20 to a more strongly p-type cadmium
telluride layer 20 having a relatively low resistivity. Additionally, the
cadmium telluride layer 20 can recrystallize and undergo grain growth
during annealing.

[0061] Annealing the cadmium telluride layer 20 can be carried out in the
presence of cadmium chloride in order to dope the cadmium telluride layer
20 with chloride ions. For example, the cadmium telluride layer 20 can be
washed with an aqueous solution containing cadmium chloride and then
annealed at the elevated temperature.

[0062] In one particular embodiment, after annealing the cadmium telluride
layer 20 in the presence of cadmium chloride, the surface can be washed
to remove any cadmium oxide formed on the surface. This surface
preparation can leave a Te-rich surface on the cadmium telluride layer 20
by removing oxides from the surface, such as CdO, CdTeO3,
CdTe2O5, etc. For instance, the surface can be washed with a
suitable solvent (e.g., ethylenediamine also known as 1,2 diaminoethane
or "DAE") to remove any cadmium oxide from the surface.

[0063] Additionally, copper can be added to the cadmium telluride layer
20. Along with a suitable etch, the addition of copper to the cadmium
telluride layer 20 can form a surface of copper-telluride on the cadmium
telluride layer 20 in order to obtain a low-resistance electrical contact
between the cadmium telluride layer 20 (i.e., the p-type layer) and the
back contact layer(s). Specifically, the addition of copper can create a
surface layer of cuprous telluride (Cu2Te) between the cadmium
telluride layer 20 and the back contact layer 22. Thus, the Te-rich
surface of the cadmium telluride layer 20 can enhance the collection of
current created by the device through lower resistivity between the
cadmium telluride layer 20 and the back contact layer 22.

[0064] Copper can be applied to the exposed surface of the cadmium
telluride layer 20 by any process. For example, copper can be sprayed or
washed on the surface of the cadmium telluride layer 20 in a solution
with a suitable solvent (e.g., methanol, water, or the like, or
combinations thereof) followed by annealing. In particular embodiments,
the copper may be supplied in the solution in the form of copper
chloride, copper iodide, or copper acetate. The annealing temperature is
sufficient to allow diffusion of the copper ions into the cadmium
telluride layer 20, such as from about 125° C. to about
300° C. (e.g. from about 150° C. to about 200° C.)
for about 5 minutes to about 30 minutes, such as from about 10 to about
25 minutes.

[0065] A back contact layer 22 is shown on the cadmium telluride layer 20.
The back contact layer 22 generally serves as the back electrical
contact, in relation to the opposite, TCO layer 14 serving as the front
electrical contact. The back contact layer 22 can be formed on, and in
one embodiment is in direct contact with, the cadmium telluride layer 20.
The back contact layer 22 is suitably made from one or more highly
conductive materials, such as elemental nickel, chromium, copper, tin,
aluminum, gold, silver, technetium or alloys or mixtures thereof.
Additionally, the back contact layer 22 can be a single layer or can be a
plurality of layers. In one particular embodiment, the back contact layer
22 can include graphite, such as a layer of carbon deposited on the
p-layer followed by one or more layers of metal, such as the metals
described above. The back contact layer 22, if made of or comprising one
or more metals, is suitably applied by a technique such as sputtering or
metal evaporation. If it is made from a graphite and polymer blend, or
from a carbon paste, the blend or paste is applied to the semiconductor
device by any suitable method for spreading the blend or paste, such as
screen printing, spraying or by a "doctor" blade. After the application
of the graphite blend or carbon paste, the device can be heated to
convert the blend or paste into the conductive back contact layer. A
carbon layer, if used, can be from about 0.1 μm to about 10 μm in
thickness, for example from about 1 μm to about 5 μm. A metal layer
of the back contact, if used for or as part of the back contact layer 22,
can be from about 0.1 μm to about 1.5 μm in thickness.

[0066] The encapsulating glass 24 is also shown in the exemplary cadmium
telluride thin film photovoltaic device 10 of FIG. 1.

[0067] Other components (not shown) can be included in the exemplary
device 10, such as bus bars, external wiring, laser etches, etc. For
example, when the device 10 forms a photovoltaic cell of a photovoltaic
module, a plurality of photovoltaic cells can be connected in series in
order to achieve a desired voltage, such as through an electrical wiring
connection. Each end of the series connected cells can be attached to a
suitable conductor such as a wire or bus bar, to direct the
photovoltaically generated current to convenient locations for connection
to a device or other system using the generated electric. A convenient
means for achieving such series connections is to laser scribe the device
to divide the device into a series of cells connected by interconnects.
In one particular embodiment, for instance, a laser can be used to scribe
the deposited layers of the semiconductor device to divide the device
into a plurality of series connected cells.

[0068] FIG. 2 shows a flow diagram of an exemplary method 30 of
manufacturing a photovoltaic device according to one embodiment of the
present invention. According to the exemplary method 30, a TCO layer is
formed on a glass substrate at 32. At 34, a resistive transparent layer
is formed on the TCO layer. A cadmium sulfide layer is formed on the
resistive transparent layer at 36, and a cadmium telluride layer is
formed on the cadmium sulfide layer at 38. The cadmium telluride layer
can be annealed in the presence of cadmium chloride at 40, and washed to
remove any oxides formed on the surface at 42. The cadmium telluride
layer can be doped with copper at 44. At 46, back contact layer(s) can be
applied over the cadmium telluride layer, and an encapsulating glass can
be applied over the back contact layer at 48.

[0069] One of ordinary skill in the art should recognize that other
processing and/or treatments can be included in the method 30. For
instance, the method may also include laser scribing to form electrically
isolated photovoltaic cells in the device. These electrically isolated
photovoltaic cells can then be connected in series to form a photovoltaic
module. Also, electrical wires can be connected to positive and negative
terminals of the photovoltaic module to provide lead wires to harness
electrical current produced by the photovoltaic module.

[0070] This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in the art
to practice the invention, including making and using any devices or
systems and performing any incorporated methods. The patentable scope of
the invention is defined by the claims, and may include other examples
that occur to those skilled in the art. Such other examples are intended
to be within the scope of the claims if they include structural elements
that do not differ from the literal language of the claims, or if they
include equivalent structural elements with insubstantial differences
from the literal languages of the claims.